The present invention relates to vulcanizable EPDM rubber blends, and, in particular to peroxide vulcanizates of a rubber blend comprising a mixture of high Mooney viscosity and low Mooney viscosity EPDM rubbers and articles of manufacture, such as mechanical seals for hydraulic systems formed therefrom. 2. Background Information
The tendency of polymeric materials used in the sealing parts for hydraulic cylinders, to wear is well known. This creates a variety of problems including a loss of mechanical properties such as low resistance to abrasion, erosion, and sealing capability. Consequently, it has been proposed to improve the mechanical properties of the sealing parts by using other polymeric materials, such as styrene-butadiene rubber (SBR). Although the use of styrene-butadiene rubber has been proposed for use in sealing parts, this use is subject to a variety of constraints. For example, such components cannot be used at higher temperatures or with high boiling substances (such as boric ester-added ethylene glycol and silicon oil) for brake fluids. As a consequence, manufacturers have now switched from traditional styrene-butadiene rubber components to rubber components comprising ethylene propylene diene monomer (EPDM).
Recently, due to the installment of electronic lockproofing mechanisms (e.g. the "ESC system") to guard against impact when braking, a need has arisen for sealing parts having great strength (particularly high resistance to erosion and to abrasion) and satisfactory sealing properties. Such sealing parts, however, do not retain these mechanical properties over long periods of time.
It has been proposed to improve sealing parts for a hydraulic cylinder comprising a composition formed of a peroxide vulcanizate of a rubber compound comprising as a polymer component an ethylene propylene diene monomer. The polymer component satisfies the following requirements: (a) an ethylene content of 60 to 70 wt%, an .alpha.-olefin content of 40 to 30 wt%, a Mooney viscosity of (ML.sub.1+4 100.degree. C.; hereinafter referred to as "ML") 35 to 55, and an iodine value of 20 to 35; (b) a molecular weight distribution where M.sub.w /M.sub.n .ltoreq.5.0; and (c) an .alpha.-olefin content in the polymer component which is 20 wt%, each on the opposite sides of the o-olefin composition distribution where C.sub.3 E-C.sub.3 I.ltoreq.5.0 wt%, where C.sub.3 I stands for the high molecular weight side .alpha.-olefin component and C.sub.3 E for the low molecular weight side .alpha.-olefin component (see Japanese Patent Application Laid-open SHO 61(1986)-221249).
The rubber compound comprising ethylene propylene diene monomer as the polymer component, however, exhibits poor kneading processability. That is, dispersal of additional (supplementary) materials in the rubber compound during the primary kneading step with a Banbury mixer or the like prior to roll milling is not satisfactory, and the composition obtained by the primary kneading does not exhibit roll retention during secondary kneading, i.e., the roll milling.
Furthermore, the peroxide vulcanizate does not possess sufficient resistance to low-temperature compression permanent strain in some cases when excellent physical properties are required.
Applicants have now devised a sealing part for a hydraulic cylinder which eliminates or substantially overcomes the aforementioned problems. This sealing part exhibits an improved resistance to low-temperature permanent compression strain and permits improvement in the kneading processability of a rubber compound used for producing the sealing part.